Sending Republicans pro–gay marriage postcards cuts their turnout on Election Day
Political scientists have grown considerably more comfortable conducting scientifically rigorous experiments in recent years, and the results have proven surprisingly useful for real-life campaigns: Canvassing works better than phone banking; snitching to neighbors about people not voting in the past increases turnout; TV ads are effective, but the effects are short-lived.
Here's another one to add to the list: In some cases, campaign communications can be designed to actually reduce turnout from people you don't want voting.
Duke grad student Ying Shi conducted an experiment, later published in the journal Political Communication, around the May 2012 primary elections in North Carolina, which featured a vote on Amendment 1, a proposed state-level constitutional amendment banning same-sex marriage.
Shi sent 9,211 individuals roughly half Democrats, half Republicans one of two messages: a promarriage equality message (including arguments like, "Denying marriage to same-sex couples is a form of discrimination" or an antimarriage equality message ("Nontraditional forms of marriage result in children growing up in an environment that often lacks a biological parent" . She then compared their turnout patterns in the primary with those of a control group.
http://www.vox.com/2016/5/30/11764840/voter-turnout-demobilization-experiment
If repukes want to depress Democratic turnout with their voter suppression schemes, two can play that game. Go for it, Dems!